The invention relates to a method for evaluating the resilience of a welded assembly, together with a measuring device applicable to this method and applied to the measurement of speeds of ultrasonic surface waves.
The resilience of a mechanical part is the energy required to rupture it. It can be evaluated by tests carried out on test pieces representative of the assembly and respecting certain standards in specialised instruments like the Charpy machine. It is evident that the manufacture of test pieces and the utilisation of a machine are restricting and that methods more easy to apply, in particular non-destructive monitoring through which the resilience could be evaluated indirectly, would be advantageous.
Such an indirect method for evaluating resilience has been designed and is the aim of this invention. It rests on using ultrasonic surface waves and is applied to a special family of parts: welded assemblies, in particular without addition of material, by diffusion or by analogous methods, of a somewhat thin sheet on a substrate which can be another sheet or a thicker part.
Ultrasonic tests are normally carried out after plunging the part to be analysed into a liquid. Ultrasounds are emitted in the liquid towards the part, and they can be reflected or absorbed by the latter when they reach it. For certain favourable angles of incidence, they can also undergo a conversion of the propagation mode and propagate to the surface of the part, without penetrating further than a shallow depth, providing surface waves called Rayleigh waves. Inventors"" research has demonstrated that these surface waves could be correlated to the quality of welding of the assemblies mentioned above, and more precisely that a parameter linked to the speed of propagation of these waves in these assemblies could have a correlation with their resilience, the welding affecting the wave propagation and this parameter in function of its quality.
In its most general form, the invention is a method for evaluating the resilience of a welded assembly of a metal sheet on a substrate, characterised in that it consists of producing ultrasonic surface waves on the sheet, to measure a speed of said waves then to deduce a speed parameter, and to deduce the resilience of the assembly according to a correlation function, obtained beforehand on calibration test pieces comprising a similar welded assembly, which links resilience of calibration test pieces to the speed parameter of calibration test pieces.
Preferably, two speeds of these waves are measured on the welded assembly, along the two principal directions of the sheet, because of its anisotropy. The inventors consider that the speed parameter should be deduced from the slower of the two speeds. This parameter can be a difference in the speed of ultrasonic waves measured on the metal sheet of the welded assembly and on a reference piece made out of the basic material of the sheet metal but without the welded assembly. Then it is suitable that the speeds for which the calculation is made are measured under the same conditions, that is to say that they involve the same measurement direction on the sheet metal of the welded assembly and the metal sheet of the reference piece.
Another aspect of the invention is a measuring instrument applicable to this method; it consists of an instrument for measuring the speed of ultrasonic surface waves on a part, comprising an emission head for waves directed towards the part but at an inclination, characterised in that it comprises a second head, destined either to collect a portion of the waves diffused from the part, or to return said portion towards the emission head as an echo, the second head being directed towards the part but with an inclination opposite to that of the inclination of the emission head, and comprising two active faces, in order to collect or to return said portion of waves, arranged in steps and at an identical distance from the part.
Advantageously, it comprises a frame and head support mechanisms on the frame, designed in such a way as to enable adjustment of the inclination of the heads; and it is recommended that the active surfaces are cylindrical with parallel axes and of the same radius.